1. Field of the Invention
The invention relates to sintered polycrystalline silicon films and somewhat more particularly to the method of producing such films by extruding a slurry or slip of fine silicon particles admixed with suitable additives into a film and sintering such film.
2. Prior Art
Copending and commonly assigned patent applications Ser. No. 160,214, filed June 17, 1980 now U.S. Pat. No. 4,330,358; Ser. No. 261,780, filed May 8, 1981 now U.S. Pat. No. 4,357,200 and Ser. No. 261,921 now U.S. Pat. No. 4,357,201, filed May 8, 1981 disclose and claim details relative to methods for producing plate-shaped or tape-shaped silicon crystal bodies which have a crystalline pillar structure equivalent to a crystalline calumnar structure and which are suitable for fabrication into solar cells. These silicon bodies are produced by sintering a silicon powder starting material whereby such bodies are drawn-out or extruded from a silicon slip (i.e., a slurry of powder-form silicon and a suitable binder) with a suitable extrusion means onto a base as a film and such film is then sintered at a temperature below the melting point of silicon or at a temperature which only briefly attains the melting point of silicon (i.e., generally in the range of about 1350.degree. to 1430.degree. C.). The above-referenced Ser. No. 160,214 particularly provides many details concerning this method of producing polycrystalline silicon films by means of drawing and sintering so that a further discussion of details which are known per se is not necessary for a description of the present invention.
Within a framework of experiments aimed at attaining improvements to the state of development already known, as described, for example, in the above-referenced patent applications, it was determined that it was not sufficient for optimum results in the production of sintered polycrystalline films to merely transfer techniques which are obtained from years-long practice in producing sintered ceramic films and which have become a set part of production steps in manufacture of sintered films.
In instances of working with a slip of silicon, which is a covalently bound element, during the course of a sintering process, the appearance of a significant grain growth can already be noted at a point of time which is before the beginning of shrinkage of the slip or body formed from such slip. As has been determined, a too early occurance of grain growth hinders a further shrinkage of material so that, as result, a sintered silicon film arises which has a relatively high degree of open porosity.
The foregoing disadvantage was, however, solved to a certain degree whereby one undertook an at least partial melting of the silicon material during the actual sintering process. A disadvantage of this solution is that a relatively complicated method for exact temperature monitoring during the time interval of the melting aimed-for is required. Further, the kiln furniture, such as support members, composed of, for example, aluminum oxide or silicon dioxide, are greatly stressed by sintering/melting temperatures.
For quite a while, generally in order to attain densely sintered materials, sintering under pressure was utilized. However, this required significant additional technological outlay and could hardly be considered for inexpensive goods, such as materials for solar cells.